Lecture 10: S-Phase & Mitosis

S-phase & mitosis Control of chromosome duplication: initiation happens only once in a cell cycle Late metaphase checkpoint There is a checkpoint at the late metaphase It ensures that all the chromosomes are attached to the spindle in a correct way Anaphase can start only after all kinetochores are corrector connected to spindle APC triggers the anaphase Ubiquitin ligase APC (Anaphase promoting complex, also called APC/C) tarts the anaphase APC is inactive until it binds to Cdc20 Each unattached kinetochore inhibits Cdc20 through checkpoint proteins Once the kinetochore is attached, the checkpoint proteins are moved away from kinetochore and Cdc20 makes a complex with APC and activates it Anaphase starts by separation of sister chromatids APC is activated by Cdc20 APC is a ubiquitin ligase Sister chromatids are kept together with cohesins Cohesins are degraded by a protease called separase Separase is inhibited by binding to securin APC adds polyubiquitin to securin, which targets it for degradation This releases the separase APC also adds ubiquitin chains to cyclins, which targets them to degradation as well Anaphase Events in anaphase Separation of sister chromatids Movement of sister chromatids to the centrosome Movements of poles towards the plasma membrane Degradation of M and S cyclins APC marks _____ for degradation • A. Cyclin M • B. Separase • C. Condensins Two movements of anaphase, associated with shrinking of some microtubules Anaphase A movement: sister chromatids move to the poles Anaphase B movement: poles move apart Two major forces of anaphase B Kinesins move poles apart (and closer to the plasma membrane) by sliding the microtubules relative to each other Dyneins move poles apart by pulling centrosomes to the plasma membrane Microtubules behavior in mitosis Interphase: microtubules are long and stable, project to the cell wall Prophase: microtubules are destabilized , more of them are formed at centrosomes, spindle poles separate Prometaphase: microtubules catch condensed chromosomes by kinetochores, once they become available upon nuclear envelope breakdown Metaphase: microtubules keep chromosomes attached to both poles through kinetochores. Chromosomes oscillate as they are pulled by microtubules to both poles Anaphase: chromosomes separate and move to the poles. Two movements associated with the shrinking of microtubules. Anaphase movement A - microtubules shorten and pull separated chromosomes to the poles. Anaphase movement B - overlapping microtubules slide and push poles apart Telophase: nuclear envelope reforms. Microtubules are back to interphase state Anaphase A and B movement happen ______ • A. At the same time • B. First A, then B Mitosis in plants Cell plate: the new cell wall between the two daughter cells Cell plate formation is guided by a phragmoplast: leftovers of overlapping microtubules between the poles Division of organelles ER is cut in two during cytokinesis; more ER is formed from existing ER Mitochondria are separated randomly with the cytoplasm during cytokinesis. It’s division mostly correlates with a cell energy needs rather than cell cycle. It occurs proportional to the cell growth in cell cycle Golgi is fragmented in mitosis and moved to the spindle poles; reconstructree in telophase